1. Field of the Invention
The present invention relates to a compact spectrophotometer that can conduct spectrophotometry, without using a prism or a diffraction grating.
2. Description of the Related Art
The conventional method for measuring spectral intensity comprises guiding light along directions which differ for each wavelength by using a prism or a diffraction grating, illuminating a linear sensor or the like, and measuring the output from each element of the linear. However, when a prism or diffraction grating is used, a certain space is necessary to guide and separate spectral components of the light along the directions which differ depending on the wavelength. Accordingly, an increased size of spectrophotometers has been a problem. Another problem has been that since the light to be analyzed passes through a slit when guided to a prism or diffraction grating, the quantity of light is decreased and the accumulation time required by the linear sensor is extended, which has made it difficult to conduct fast measurements.
Several methods using a transmitted wavelength variable filter (referred to as a Linear Variable Filter or LVF hereinbelow) have been suggested and used in practice to resolve the above-described problems. One such method is disclosed in Japanese Patent Application No. H05-322653 and is illustrated in FIG. 11.
As shown in FIG. 11, a linear sensor 21 is equipped with a Linear Variable Filter 22 which is formed fixedly on a linear sensor 24 so as to sandwich a resin 23 such as an organosilicon compound. The Linear Variable Filter 22 comprises an interference filter in which a transparent electrically conducted film is formed with a thickness differing depending on location, as shown in FIG. 11, and therefore the transmitted wavelength differs depending on location.
Changing successively the thickness of the transparent electrically conductive film formed on the linear sensor 24 correspondingly to unit elements thereof provides for spectral separation of light of different wavelength bands and reception thereof by unit elements of linear sensor 24. With such a method, a spectral film and a photoelectric converter are integrated and a size is obtained which is about the same as that of a usual linear sensor. For this reason, such linear sensors featuring small size and weight and low cost have been marketed.
A spectrophotometer using a Linear Variable Filter of another system is disclosed in U.S. Pat. No. 5,872,655, and has also been marketed. In the Linear Variable Filter used therein, a dielectric material with a low dielectric constant and a dielectric material with a high dielectric constant are formed alternately as films on a substrate in vacuum by using an IAD (Ion Assisted Deposition) method, the number of layers being no less than 200. Changing the film thickness in the longitudinal direction of the substrate provides for characteristic such that the transmitted wavelength changes linearly according to the position in the longitudinal direction.
FIG. 12 illustrates the structure of the system disclosed in U.S. Pat. No. 5,872,655. As shown in FIG. 12, Linear Variable Filter 25 is formed as a film on a substrate 26 by the above-mentioned IAD method. A bandpass filter 27 is attached to the opposite side of substrate 26. The Linear Variable Filter 25 is attached to a linear sensor 28, and if light falls from the side of bandpass filter 27, the wavelength of the light that passed through the Linear Variable Filter 25 will differ depending on position, the light of different wavelength will fall on elements of linear sensor 28, and a spectrophotometer in which a spectral film and a photoelectric converter are integrated can be realized.
Another spectrophotometer using a Linear Variable Filter of a type different from the above-described types is disclosed in U.S. Pat. No. 6,057,925, and has also been marketed. In this spectrophotometer, an optical system forming an upright, noninverted image is inserted between a Linear Variable Filter and a linear sensor. As a result, light beams separated into spectral components and propagating from the Linear Variable Filter form an image on the linear sensor, and a GRIN (Gradient Index) lens or Micro Lens Array is used as a compact system for forming the upright, noninverted image.
FIG. 13 illustrates a structure using a GRIN lens. In this structure, transparent glass sheets 32, 33 are attached to a Linear Variable Filter 31 and then a GRIN lens 34 is adhesively bonded and integrated therewith.
The sensor surface of a linear sensor 36 is arranged so as to be in a position at a prescribed distance from the GRIN lens 34 via a transparent glass sheet 35. The thickness of transparent glass sheet 33 and the distance L between the GRIN lens 34 and the photosensitive surface of linear sensor 36 serve as conditions for forming the upright, noninverted image.
However, the system disclosed in Japanese Patent Application Laid-open No. H05-322653 which is illustrated in FIG. 11, and the method disclosed in U.S. Pat. No. 5,872,655 which is illustrated in FIG. 12 have the following drawbacks. Namely, in both cases, structures are obtained in which a Linear Variable Filter is attached to a linear sensor. As a result, multiple reflections occur between the Linear Variable Filter and the linear sensor and the spectral characteristic is degraded.
The method disclosed in U.S. Pat. No. 6,057,925 illustrated in FIG. 13 resolves those problems, but another problem is associated therewith. Namely, the GRIN lens is composed of a total of 28 rod-like lenses 37 arranged in two rows as shown in FIG. 14. Therefore, if a surface image such as that of the Linear Variable Filter 31 is projected, a synthesized image produced by 28 rod-like lenses is formed on the linear sensor 36. Therefore, strictly speaking, 28 peak distortions appear in the output of linear sensor 36. As a result, even if the accuracy of the position of the spectra is increased the accuracy of the magnitude of the output is decreased.
In particular, differentiation of optical spectra is often conducted in the application field of spectrophotometers. However, in such cases, the inaccurate magnitude of output results in much noise and the accuracy of values after the differentiation is degraded. It therefore becomes impossible to conduct differentiation from the optical spectra.
Moreover, though the image is upright and noninverted, since the image is formed by 28 rod-like lenses with a small surface area and the image forming distance is much larger than that in the above-mentioned conventional systems, the light transfer ratio decreases to no more than 2-3%. As a result, the capability of shortening the scanning period of linear sensor 35 and measuring high-speed phenomena provided for by the utilization of a large quantity of light, which is inherent to spectroscopes with a Linear Variable Filter, is lost.